This invention relates to a new system for processing communications signals using optical-frequency multiplexing in communicating through optical fibers and, also, to various novel processes and novel apparatus used in the system. More particularly the system utilizes stimulated scattering energy as a means to improve tunability, power levels, and channel discrimination of such systems.
The advantages of utilizing optical fibers in communications systems have long been recognized. More recent experience with such fibers has confirmed their value and encouraged their use in data-transmission systems of increasing complexity and information-bearing capability.
Thus, optical-frequency multiplexing of signals has been recognized as a promising way to allow high rates of data transmission through an optical fiber, and, also, as a way to provide a means from which individual dual channels of information can be selectively extracted by appropriate means along the fiber. Such systems practically, but not necessarily, utilize single-polarization or polarization-maintaining fiber.
It has been a continuing problem to meet the potential of fiber-optic systems by providing means to allow more signals to be multiplexed in a single fiber and, particularly, to provide means for selectively and economically tapping into and out of such multiplexed systems for sending and receiving selected channels of the information being transmitted therein. The present inventor has described various problems and solutions relating to these very general problems in U.S. Pat. Nos. 4,315,666 and 4,342,499 and in an International Application published under the Patent Cooperation Treaty (PCT) No. WO83/02168. U.S. Pat. No. 4,315,666 relates to coupling of fibers to transmit signals therebetween by varying the shape and placement of coupled cores in fiber constructions; U.S. Pat. No. 4,342,499 relates to means for transferring any of a number of wavelengths from one fiber to an adjacent fiber by such physical means as, for example, bending or tapering coupled fiber cores to change the wavelength at which tuning occurs. The PCT publication discloses means to separate specific wavelengths from a broader spectrum of wavelengths through use of a novel fiber optics device which is mechanically distinct from the optical fiber carrying the broader spectrum of signals. This novel device, called a resonant-cavity signal tap, was developed in response to the need to be able to selectively receive and interpret individual signal channels without disturbing the much larger number of signal bearing channels which optical-communications designers wish to use in communications systems.
A problem relating to such resonant cavity taps, as described in the PCT application, is inherent in the fact that there are rather long tails on the transfer curves of such devices. Some alleviation of this problem is achieved by using a number of such taps in series; but, the maintenance of channel (frequency) spectral width when more than one tap is used in series requires that the transfer curve of each tap be made somewhat wider; so, the aforesaid tail on a transfer curve becomes even longer, thereby limiting the utility of the resonant cavity tap even when several area used together to improve channel discrimination.
For example, were one to attempt to transmit video width channels (about 10.sup.8 pulses per second is required for digital, 500-line, three-color video transmission), the resonant-cavity tap system would present very substantial difficulties in making the resonant cavity small enough to accomodate the required spectral width and still provide suitable discrimination between channels to facilitate the clear reception of a desired channel.
There are a number of other constraints in the systems of the prior art, particularly those aspects of a communications system relating to the means for accomplishing a wavelength-selective reception, or transmission of a specific signal from, or to, the multiplexed single-fiber communications lines, e.g. trunk lines. As will be seen below, the inventor, has directed his efforts to solve such problems and make other improvements in such systems.
It has also been proposed in the prior art that heterodyning can be utilized as a technique for detecting and separating one signal from a group of signals being transmitted along a fiber. In general, this heterodyning technique comprises adding a strong unmodulated, spectrally narrow (heterodyne) signal to the group of signals, detecting this mixture, and electronically filtering out a beat frequency corresponding to the difference in optical frequency between the heterodyne frequency and the channel being selected. In general, heterodyne-aided discrimination, as contemplated in the prior art, has been adequate to select one channel from a set of only about 10 to 20 channels.
Other published art, not heretofore related to solution of problems addressed by the inventor, describes some of the characteristics of Raman scattering and, more particularly, Brillouin scattering in optical fibers. (Ippen et al, pages 539-541, Appl. Phys. Lett., Vol. 21, No. 11 December 1972). Brillouin scattering also has been suggested for use in operation of lasers. An example of such a laser is discussed in U.S. Pat. No. 4,530,097 to Stokes.
Other publications relating to multiplexing or coupling in optical communications include articles by Tomlinson's Applied Optics, 16, pp. 2180-2194, August 1977; by Taylor in Optics Communications 8, pp. 421-425 August 1973; in Applied Optics 17, pp. 3253-3258; in Fiber and Integrated Optics 1, pp. 227-241 (1978); and Bell System Technical Journal pp. 2103-2132 September 1969.
This discussion of the background is necessarily made with full knowledge of the inventions disclosed herein. It is to be understood that the discussion herein of the various categories of prior art is in no way to be taken as an admission that anyone, before the present invention was made, has related the various aspects of the art one to another in addressing any problems addressed by the present inventor.